1. Field of the Invention
This invention relates to a method for generating power with a gas turbine using the gas resulting from gasification of a fuel of inferior quality such as coal, tar, or less volatile oil by the use of a pressure swing method for the production of oxygen.
2. Description of the Prior Art
The coal gasification composite power generation system can be expected to offer highly efficient and clean, power generation because it utilizes the gas turbine which is the most efficient of all the heat engines using fossil fuels and because it uses as the fuel for the gas turbine the gasified fuel which has been produced by a gasifying furnace and scrubbed to expel dust and desulfurized. For the gasification of such fuels of inferior quality as coal, tar, and less volatile oil, methods using air and methods which use oxygen respectively as an oxidizing agent have been proposed.
The use of air in this gasification is disadvantageous from the standpoint of motive power because the gasifying furnace for the gasification of such a fuel of inferior quality is operated under a pressure higher than the pressure of the gas turbine and, as a result, the nitrogen which accompanies oxygen in the air is inevitably destined to increase in pressure. Further, since the gasified fuel obtained by this gasification falls short of 1,000 Kcal/Nm.sup.3 in calorific value, the combustion of this gasified fuel calls for skill. Further, since the gasifying furnace itself and the cleanup system for removal of dust and desulfurization depend largely on the amount of a gas to be treated, the method which uses air entrained by the nitrogen originating in the air incurs an unduly large cost of equipment.
For the reason given above, the methods using oxygen muster expectations. In reality, however, the method of deep cooling separation which is widely used for the production of oxygen in a large volume has not overcome the drawback inherent in the use of air because it entails a heavy power consumption for the production of oxygen and incurs a high cost of equipment. Moreover, the deep cooling method encounters unusual difficulties in coping with the practices of load change, weekend interruption, and daily interruption which have found acceptance in the power generation systems in recent years.
FIG. 2 is a schematic diagram of an oxygen blowing coal gasifying composite power generation system possessing such merits and demerits as described above. The air which has been compressed to a pressure of about 6 atm by an air compressor 01 is separated in a total low-pressure type deep cooling separation and production device 02 into oxygen gas 03 of a pressure in the range of 1 to 1.2 atms and nitrogen gas 04 of a pressure in the range of 1 to 1.2 atms. The oxygen obtained in the total low-pressure type deep cooling separation and production device 02 has a very high concentration falling in the range of 90 to 99.6 vol %. The power during the production of oxygen is approximately 0.36 kwh/Nm.sup.3 --O.sub.2 in the case of an oxygen purity of 90 vol % and 0.4 kwh/.sup.3 --O.sub.2 in the case of an oxygen purity of 99.6 vol %.
The power generation system does not require the oxygen to be in a very high concentration and, therefore, effects gasification of coal by producing oxygen gas in a purity of 90 vol %, compressing the oxygen gas to a pressure of 30 atms. by an oxygen compressing device 05, mixing the compressed oxygen gas with steam 06, and supplying the resultant mixture as accompanied by coal 08 to a coal gasifying furnace 07. The resultant gasified coal having an average calorific value of 5,000 Kcal/Nm.sup.3 is scrubbed to expel dust by a high-temperature dust removing device 09, is desulfurized by a dry type desulfurizing device 010 to be cleaned enough to be used in the gas turbine cycle, and then is supplied via a flow path 011 to a combustion device 012.
In the meantime, the greater part of the air which has been compressed by a compressor 013 is fed to the combustion device 012, consumed therein for effecting combustion of the aforementioned gasified coal in a compressed state, and is introduced in the form of a hot compressed fluid via a flow path 015 into an inflation type gas turbine 016. On the existing technical level, the highest allowable working temperature of the material of the gas turbine is about 1,380.degree. C. Since the temperature of combustion in the combustion device 012 exceeds this temperature, part of the aforementioned compressed air is supplied via a flow path 017 to the flow path 015 and used for cooling the combustion device 012.
The compressor 013 and the gas turbine 016 are connected with a common shaft 018 to a power generator 019 so that the rotation of the gas turbine 016 produces electric power. Since the exhaust gas which emanates at a lower temperature under a lower pressure from the gas turbine 016 after performing its work therein has a temperature of about 500.degree. C., it is treated in a waste heat recovery boiler and steam turbine system 020 for collection of electric energy.
The power generation system which uses oxygen in place of air as an oxidizing agent in the gasification of such a fuel of inferior quality as coal as described above operates by the use of a relatively small amount of oxygen separated from nitrogen and, therefore, enjoys many prominent advantages such as:
(1) Generous reduction in the power required for increasing the pressure of the oxidizing agent in the gasifying furnace and economization of the cost of the pressure increasing device.
(2) Reduction in the cost of equipment due to a decrease in the cross-sectional area of the coal gasifying furnace.
(3) Reduction in the equipment cost and variable cost of the dust removing device and the desulfurizing device, which both depend largely on the amount of gas subjected to the treatment, due to a decrease in the volume of the gasified coal.
(4) Improvement of the operating capacity of the combustion device due to an increase in the calorific value of the gasified coal.
(5) Simultaneity of process between the power generation and the gasification for production of chemical raw material.
The fact that the equipment cost and the variable cost for the method of deep cooling separation, which is widely employed as means for the production of oxygen in a large volume, are such as to offset substantially the aforementioned advantages derived from using oxygen as the oxidizing agent for gasification. And the fact that such operations as load change, weekend interruption, and daily interruption, which and indispensable requirements for the power generation system, are extremely difficult to effect are causes for reluctance shown in the adoption of this method.